The present invention relates to an apparatus and method for measuring a changing amount of insertion loss of an optical device depending on a polarization state of an incident light, i.e., a polarization-dependent loss.
Due to development of very high-speed optical communication technology, various optical elements are being developed. In order to effectively manage qualities of the optical elements, the optical communication industry requires optical measuring instruments having high reliability and high measuring speed. In particular, as introduction of Wavelength Division Multiplexing (WDM) transmission method promotes the necessity of measuring the features of the optical devices with respect to various wavelengths across wide wavelength ranges, the requirement is also increasing for optical measuring instruments having high measuring speed.
Among various specifications of the optical elements, Polarization-Dependent Loss (PDL), in particular, is characterized in that measurement thereof is difficult and the measuring time is long. Moreover, PDL is one of major features of the optical devices which require strict quality control since it can cause degradation of signal as optical communication increases in speed.
Insertion loss or PDL of optical device is defined from the ratio of the intensity of an input optical signal to the intensity of an output optical signal, and depends on polarization of an incident light. PDL representing such features is generally measured as follows: First, an incident light is introduced into a Device Under Test (DUT) subject to PDL measurement while the polarization of the incident light is varied and the intensity thereof is maintained constant. Then, the intensity of an output light is measured. PDL is obtained from the ratio of the maximum output Pmax to the minimum output Pmin as defined in Equation 1:                     PDL        =                  10          ·                      log            ⁡                          (                                                P                  max                                                  P                  min                                            )                                ·                                    Equation        ⁢                  xe2x80x83                ⁢                  1                    
Conventional methods for measuring PDL defined as above include all state scanning method and Mueller matrix method. The all state scanning method forms polarization states of an incident light as various as possible and measures the intensity of an output light so as to obtain the maximum output value and the minimum output value, and description thereof is as follows.
FIG. 1 shows the structure of an apparatus for measuring PDL according to the all state scanning method. Referring to FIG. 1, a laser diode 100 has an output signal with a predetermined polarization state and intensity. The output signal from the laser diode 100 primarily propagates along a polarization adjuster 110, in which the polarization adjuster 110 is comprised of a few waveplates made of optical fiber. Changing the angles of the waveplates allows adjustment of the polarization state of an incident light entering a DUT 130 via an optical fiber 120. An optical power meter measures the intensity of the output optical signal from the DUT 130. In other words, the waveplates of the polarization adjuster 110 are scanned in the angles so that the input optical signal has all polarization states after propagating along the DUT 130, and then the maximum and minimum values of the polarized output optical signal are extracted for a predetermined time period so as to calculate PDL values. However, this apparatus has the worst disadvantage that mechanical polarization adjusters are generally used so that a measuring time is prolonged too long up to about 5 to 10 seconds thereby causing the apparatus rarely available at manufacturing sites.
In the meantime, the Mueller matrix method obtains the maximum output value and the minimum output value through mathematical calculation using output values about four correctly known input polarization states, in which a detailed measuring method thereof is disclosed in U.S. Pat. No. 5,371,597 granted to Favin et al. According to U.S. Pat. No. 5,371,597, polarization adjusters including manual and automatic polarization adjusters are used in order to obtain the known four polarization states of an incident optical signal at the leading end of a DUT. Available examples of the automatic polarization adjusters include xc2xd and xc2xc waveplates, which are rotated to adjust polarization. In performing this method, however, there are restrictions that a calibration process is inevitable for determining the ratio between the intensity of the incident optical signal and the intensity of an output optical signal of the DUT with respect to each of the four polarization states and the measuring process requires correctly obtaining resultant values of the four polarization states after inputting the same. For the purpose of this, it is essential that the input polarization states have no disturbance, however, there is a problem that the disturbance inevitably takes place in the input polarization states if the measuring process is carried out for a long time period.
It is a technical object of the present invention to provide an apparatus and method for measuring PDL capable of reducing the probability of disturbance occurrence from polarization of an incident light by completing the measurement of a polarization-dependent loss within a relatively short time.
It is another object of the present invention to provide an apparatus and method which can rapidly measure PDL according to an all states scanning method by using polarization modulators having a modulation speed of several hundreds kHz or more.
To accomplish the above objects, there is provided an apparatus for measuring a polarization-dependent loss. The apparatus comprises:
(a) a light source;
(b) a polarizer for converting a light irradiated from the light source into a polarized light;
(c) a polarization scrambler for modulating polarization state of the polarized light with a frequency F, the polarization scrambler including:
(c-1) optical fiber birefringence modulators including at least three cylindrical piezoelectric elements and optical fibers respectively wound around the outer walls of the piezoelectric elements without intermission, and
(c-2) AC voltage sources synchronized to a common clock with respect to each of the optical fiber birefringence modulators for applying an AC voltage having a frequency corresponding to the multiple of an integer relatively prime with respect to a predetermined frequency F to the each optical fiber birefringence modulator;
(d) a photodetector for, when the output light of the polarization scrambler passes through an object under test, detecting the optical power of the output light undergoes the object;
(f) an ADC synchronized to the common clock, for providing an intensity profile of the output light with the period of 1/F; and
(g) a digital signal processing unit for averaging the periodic intensity profile of the output light from the ADC to restrain a noise accompanying to each measurement.
According to another aspect of the invention, there is provided a method for measuring a polarization-dependent loss. The method comprises the steps of:
preparing a polarized incident light;
inputting the incident light into a polarization scrambler and outputting a polarization-scrambled output light with a predetermined frequency F, wherein the polarization scrambler comprises: optical fiber birefringence modulators including at least three cylindrical piezoelectric elements and optical fibers respectively wound around the outer walls of the piezoelectric elements without intermission, and AC voltage sources synchronized to a common clock with respect to each of the optical fiber birefringence modulators for applying an AC voltage having a frequency corresponding to the multiple of an integer relatively prime with respect to the predetermined frequency F to the each optical fiber birefringence modulator;
passing the light outputted from the polarization scrambler through an optical device under test;
detecting an output of the light that has passed through the optical device using a photodetector; and
averaging detected values of the photodetector with respect to birefringence modulation having a constant period to calculate the polarization-dependent loss from a ratio of the maximum output and the minimum output with respect to the period.